Austenitic stainless steels, for example, such as SUS 304, SUS 316 and SUS 310S and others, which are regulated in JIS G 4304 (2005), are known to be materials containing Cr and Ni as main elements and also excellent in corrosion resistance.
And, it is known that when austenitic steels contain P, which is essentially an impurity element, at high concentration levels, P contributes toward precipitation hardening by making M23C6 carbides finer, hence lead to improvements in high temperature strength characteristics, typically creep strength, as shown in the Patent Documents 1 to 4.
Similarly, the Patent Document 5 proposes an austenitic stainless steel markedly improved in high temperature strength characteristics, typically creep strength, as a result of its containing P at a high concentration level.
The Patent Document 6 and the Patent Document 7 disclose metallic materials which contain P at a level of exceeding 0.05% and contribute improvements in the metal dusting resistance of base metals.
The Patent Document 8 also discloses a welded joint and a welding material which is excellent in metal dusting resistance and is reduced in susceptibility to weld solidification cracking as a result of their containing Ti, at an appropriate level, which is based on a relationship between the contents of Si and Cu.
Further, the Patent Document 9 discloses, as a welding material for austenitic alloys, a welding material for a gas shielded arc welding which is reduced in susceptibility to solidification cracking by imposing the following restrictions on the contents of P and S as impurities: “P: not more than 0.01% and P+S: not more than 0.02%”. The Patent Document 10 discloses a method of welding high Cr and high Ni alloy members while inhibiting weld solidification cracking in which the method comprises using a welding material containing Nb in addition to the composition of the base metal members.
The Patent Document 11 discloses a high C and a high Cr-high Ni type welding material which is so constituted that a Cr—Ni containing alloy plate suitable for wire drawing is used as a sheath material, in which a high C and high Cr alloy powder is covered to form a base wire material.
Further, the Patent Document 12 discloses a welded joint, excellent in resistance to high temperature cracking in welding, and characterized in that the weld metal satisfies two formulas. The quantity of the inclusions, which include P, the weld metal is regulated to 0.01 to 0.1% in terms of the area ratio so that the problem of hot cracking encountered in the dissimilar material welding of SUS 304 to invar alloy (36 Ni and the balance is Fe) may be overcome.
However, since P is hardly soluble in the austenite, P causes marked increases in the susceptibility to weld solidification cracking to a larger extent, not only in the case that a primary crystal austenite solidification occurs but also particularly in the case that a fully austenitic solidification occurs.
The said weld solidification cracking occurs when the liquid phase, which exists among dendrites during weld solidification and remains in a filmy form in low temperature regions, and cannot withstand the applied stress.
An increased P content results in an increase in the susceptibility to weld solidification cracking. This is due to P being markedly concentrated in the liquid phase during solidification and therefore greatly lowers the solidification completion temperature of the liquid phase and thus the liquid phase remains in low temperature regions. Therefore, for example in the Non-Patent Document 1, it is shown that the P content should be rigidly restricted.
Consequently, in the present state of affairs, the restraints are imposed on the P content in stainless steels and Ni based alloys which show fully austenitic solidification. For example, in the case of SUS 310S which is a steel species representative of the above-mentioned austenitic stainless steels, shows the fully austenitic solidification and is regulated in JIS G 4304 (2005), the content of P is restricted to not more than 0.045%. The welding material to be used in welding the said SUS 310S, the content of P is restricted to a lower level, that is to say, not more than 0.030%, in JIS G 4316 (1991) from the viewpoint of the said weld solidification cracking.
That is to say, in spite of the fact that P is so far known to produce favorable effects on the characteristics of austenitic steels, improving high temperature strength, typically creep strength, and improving corrosion resistance and so on, the content thereof has so far been restricted since it markedly increases their susceptibility to weld solidification cracking.
Patent Document 1: JP 37-17113 A
Patent Document 2: JP 62-243742 A
Patent Document 3: JP 62-243743 A
Patent Document 4: JP 62-267454 A
Patent Document 5: WO 2006/106944
Patent Document 6: JP 2007-186727 A
Patent Document 7: JP 2007-186728 A
Patent Document 8: JP 2006-45597 A
Patent Document 9: JP 7-60481 A
Patent Document 10: JP 9-52194 A
Patent Document 11: JP 11-285890 A
Patent Document 12: JP 2001-18089 A
Non-Patent Document 1: Y. Arata, F. Matsuda and S. Katayama: Transactions of JWRI, Vo. 6-1 (1977), pp. 105 to 116